Synthetic Pt-Fe(OH) x catalysts by one-pot method for CO catalytic oxidation

Catalytic oxidation is used to control carbon monoxide (CO) emissions from industrial exhaust. In this work, The prepared Pta-Fe(OH) x catalysts (x represents the mass fraction of Pt loading (%), a = 0.5, 1 and 2) by the one-pot reduction method exhibited excellent CO catalytic activity, with the Pt2-Fe(OH) x catalyst, 70% and ∼100% CO conversion was achieved at 30°C and 60°C, respectively. In addition, the Pt2-Fe(OH) x catalyst also showed excellent H2O resistance and hydrothermal stability in comparison to the Pt2/Fe(OH) x catalyst prepared by impregnation method. Characterization results showed that the excellent catalytic performance of the catalysts was mainly attributed to the abundant surface oxygen species and Pt0 the presence of H2O, which promoted the catalytic reaction of CO, and Density functional theory (DFT) calculation showed that this was mainly attributed to the catalytic activity of the hydroxyl (-OH) species on Pt2-Fe(OH) x surface, which could easily oxidize CO to -COOH, which could be further decomposed into CO2 and H atoms. This study provides valuable insights into the design of high-efficiency non-precious metal catalysts for CO catalytic oxidation catalysts with high efficiency.


Introduction
Pt-based catalysts exhibit catalysts with excellent CO catalytic oxidation performance, but their performance usually tends to decrease significantly in the presence of H 2 O (Lou and Liu, 2017;Li et al., 2024).Fe-based catalysts are a kind of non-precious metal CO catalytic oxidation catalysts with excellent performance (Zhang et al., 2015;Zheng et al., 2016;Haneda et al., 2017;Ma et al., 2020).The composite of Pt and Fe usually also exhibits excellent catalytic performance, Li et al. (2019) loaded Pt and Fe onto mesoporous Beta molecular sieves for the catalytic oxidation of CO, and the results showed that the Pt/(Fe 3 )-mBeta catalysts showed the best catalytic activity, which was able to oxidize the CO completely at 90 °C.Most importantly, the catalysts also showed excellent water resistance and stability, which was mainly attributed to the synergistic effect between the Pt/Fe valence and the large specific surface area of the catalysts.The self-inhibition of Pt metal particles by CO seriously impaired the catalytic oxidation performance of Pt-based catalysts, and the use of reducible metal oxides to load Pt metal particles is an effective method to avoid Pt self-inhibition and improve the catalytic performance.Li et al. (2020) prepared a non-homogeneous Pt/Fe 3 O 4 catalyst by in situ reduction of chloroplatinic acid on commercial Fe 3 O 4 powders in ethylene glycol solution.Pt/Fe 3 O 4 catalysts were prepared in ethylene glycol solution with non-homogeneous structure, and the nonhomogeneous Pt/Fe 3 O 4 catalysts had better CO catalytic oxidation performance compared with Fe 3 O 4 .For the Pt/Fe 3 O 4 catalysts, the temperatures were 260 °C and 290 °C for 50% and 90% CO conversion, respectively, while for the Fe 3 O 4 catalysts, the CO could only be completely oxidized when the temperature was higher than 310 °C.The results showed that the Pt/Fe 3 O 4 catalysts had better CO oxidation performance than the Fe 3 O 4 catalysts.Characterization results showed that the metal Pt atoms have a strong synergistic effect with the Fe 3 O 4 carrier, and the Pt metal particles promote the release of lattice oxygen and the formation of oxygen vacancies on Fe 3 O 4 , and the activation of oxygen molecules on the oxygen vacancies is the decisive step in the catalytic reaction.The effect of metal Fe, Co, Ni, Cu and Ce doping on the water resistance of Pt/Al 2 O 3 catalysts was investigated by Tomita et al. (2014), and the results showed that iron oxide was the most effective promoter between −40 °C and 150 °C.The maximum CO conversion was observed at temperatures lower than 120 °C with a molar ratio of 1 to Pt, and the CO conversion increased monotonically with Fe content at temperatures higher than 120 °C.X-ray absorption finestructure analysis showed that the formation of Pt-Fe alloys enhanced the CO oxidation activity at high temperatures but decreased it at low temperatures.Zhu et al. (2021) loaded Pt onto CoFe 2 O 4 and NiFe 2 O 4 spinel catalysts for the catalytic oxidation of CO, and found that the catalytic performance of Pt loaded onto spinel carriers was significantly better than that loaded onto single-metal-oxide carriers (e.g., Co 3 O 4 , Fe 2 O 3 , and NiO carriers), and the best catalytic activity was found with the TOF ranging from 0.27 to 1 s −1 at 50 °C, which was higher than that of Pt/ Fe 2 O 4 , which is three times higher than that of Pt/Fe 2 O 3 .The results showed that the strong interaction between Pt and spinel promoted the activation of oxygen species and weakened the adsorption of CO on Pt atoms, and the catalysts also showed good water resistance, even when the reaction conditions contained 10 vol% of H 2 O vapour, the catalysts still maintained good catalytic performance, which is promising for the application of the catalysts.Wang et al. (2019) prepared carriers with different Cr/Fe ratios and loaded Pt for the catalytic oxidation of CO, and Pt/Cr 1.3 Fe 0.7 O 3 showed the best catalytic activity (conversion frequency of 0.2 s −1 at 80 °C for 1% CO + 1% O 2 ), which was mainly attributed to its best reducibility.In addition, the catalyst remained highly active in the presence of 10% CO 2 and 10% H 2 O vapor, and the kinetic results showed that CO 2 and H 2 O have different effects on the catalytic reaction, with CO 2 competing with CO for adsorption and leading to the formation of carbonates, whereas H 2 O promotes the decomposition of the carbonates, and that the promotion of H 2 O is mainly due to the decrease in the strength of CO adsorption and the decrease in CO adsorption with CO, while H 2 O promotes carbonate decomposition.In addition, the promotion of H 2 O is mainly attributed to the weakening of CO adsorption and the rapid interfacial reaction between CO and the surface hydroxyl groups formed by the dissociation of H 2 O.In addition, Pt-Fe-based catalysts are also commonly used for the preferential oxidation of CO under hydrogen-rich conditions.Prepared PtFe/CeO 2 catalysts for the preferential oxidation of CO using a one-pot method, and the Pt nanoparticles with a size of 2.8 nm were able to achieve CO conversion of 99.6% and CO 2 selectivity of 92.3% at room temperature, which meet the requirements for the preferential oxidation of CO under hydrogen-enriched conditions in fuel cell devices.The Pt nanoparticles were 2.8 nm in size.
A large number of research results have confirmed the existence of metal-oxide interfacial synergism between oxide carriers and noble metal nanoparticles, and Chen et al. ( 2014) constructed a Fe 3+ -OH-Pt interface on the surface of Pt-Fe(OH) x composite nanoparticles, which significantly improved the CO oxidation activity of Pt-Fe(OH) x compared with that of traditional Pt metal nanoparticle catalysts.Density functional theory (DFT) calculation was carried out to investigate the mechanism of catalytic promotion at the Fe 3+ -OH-Pt interface, and it was found that once CO was adsorbed on the Pt sites at the interface, it could be coupled with the adjacent OH to produce CO 2 after rapid dehydrogenation, which indicated that the OH at the interface was the active species for CO oxidation, after the desorption of CO 2 , low-covalent Fe with ligandunsaturated sites was generated at the interface, which could easily be adsorbed and activated.After the desorption of CO 2 , low valent Fe is generated at the interface with unsaturated ligands, and these Fe sites can easily adsorb and activate O 2 , and the activated oxygen species can oxidize the CO molecules adsorbed at the neighboring Pt GRAPHICAL ABSTRACT sites, and recover to the original Fe 3+ -OH-Pt active interface with the aid of water vapour, so that the process can be recycled continuously.
In this work, the Pt and Fe species exhibited a good CO catalytic oxidation performance, and the hydroxyl species may have a favorable effect on the CO catalytic reaction.Based on the above results, Pt-Fe(OH) x rich in hydroxyl species was prepared by the in situ reduction one-pot method and used for the CO catalytic oxidation.The experimental results show that the catalysts not only have excellent catalytic performance, but also have good water-resistant performance.

Experimental section 2.1 Catalysts preparation
Preparation of Pt-(OH) x catalysts with hydroxyl species on the surface by one-pot method: add a fixed amount of Fe(NO 3 ) 3 •9H 2 O and H 2 PtCl 6 into deionized water, stir to dissolve and mix well; while stirring, add NaOH solution (0.05 mol•L −1 ) drop by drop into the above mixed solution, and stop the addition of NaOH solution (0.05 mol•L −1 ), and then add 10 mL of KBH 4 (0.01 mol•L −1 ) solution drop by drop to the above solution, and continuously stir for 3 h; centrifuge the suspension, wash and vacuum dry at 80 °C; place the dried solid in an oven and dry at 150 °C for 2 h to obtain Pt a -Fe(OH) x , and the loadings of Pt metal (a = 0.5%, 1% and 2%), respectively.
Preparation of contrast Pt/Fe(OH) x catalyst by impregnation method: A quantitative amount of Fe(NO 3 ) 3 •9H 2 O was added into deionized water, dissolved and mixed homogeneously with stirring; While stirring, NaOH solution (0.05 mol•L −1 ) was added dropwise into the above mixed solution, and the addition was stopped when the pH was 9, and the stirring was continued for 3 h.The suspension was centrifuged and washed; The suspension was centrifuged, washed and vacuum dried at 80 °C.The dried solid was placed in an oven; The dried solid was placed in an oven and dried at 150 °C for 2 h to obtain Fe(OH) x carrier, and then was added to H 2 PtCl 6 solution, stirred for 1 h, then added 10 mL of KBH 4 (0.01 mol•L −1 ) solution drop by drop, and stirred continuously for 3 h; the suspension was centrifuged, washed and dried at 80 °C.The dried solid was placed in an oven; The dried solid was placed in an oven and dried at 150 °C for 2 h to obtain the Pt/Fe(OH) x catalyst.

CO catalytic oxidation activity evaluation
All evaluation experiments of the catalysts for CO catalytic oxidation were measured in a continuous flow fixed-bed reactor.To evaluate the effectiveness of internal diffusion, the Weisz-Prater Criterion was utilized.The CO catalytic oxidation activity was assessed using the standard reaction conditions: 50 mg catalyst, 1 vol% [CO], 21 vol% [O 2 ], N 2 as balance gas at a total flow rate of 30 mL•min −1 .The reactants and products were collected and examined using an N-2000 workstation and a GC9310 gas chromatograph, which was outfitted with a TDX-01 column and a TCD detector.The conversion of CO (X CO ) was calculated using Eq.1: where S CO-outlet and S CO-inlet are the CO concentrations in the inlet and outlet gas streams.

Catalyst characterization
The techniques, such as Transmission electron microscopy (TEM), H 2 /NH 3 -Temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) are carried out to characterize the obtained catalysts.Detailed description of the characterization procedure is in Supplementary Material.

DFT calculation details
The density functional theory calculations in this chapter were performed using the Vienna ab initio simulation package (VASP) software package, which describes the electron-ion interactions using the projected affixed wave (PAW) pseudopotential method.The truncation energy is 400 eV, and the exchange-correlation generalization is the GGA-PBE generalization.The description of the Fe 3d orbital electrons by the GGA generalization suffers from self-interaction errors, so it is corrected by the DFT + U method, with the value of U determined from the forbidden band width of Fe 2 O 3 (Zhao et al., 2017).It was found that the calculated forbidden bandwidth of Fe 2 O 3 is 2.02 for a U value equal to 4.6 eV, which agrees with the experimental value of 2.1 (Onoe et al., 2007), so the U value of 4.6 eV was used.For the spin-polarization calculations the magnetic properties of Fe were taken into account, and the use of a different spin orientation for Fe can have a significant effect on its energy, and all energy calculations were performed using an antiferromagnetic structure for the Fe atoms (Barnard and Guo, 2011).The structure chirality was calculated using a convergence criterion of all interatomic energies less than 10 −4 eV and forces less than 0.05 eV.Brillouin zone integrals were calculated using the Monkhorst-Pack method using 3 × 2 × 1 K points.The lowest energy reaction path was calculated using the CI-NEB method, confirming the transition state structure by having only one imaginary frequency.A vacuum layer of 15 Å was set up to avoid interactions between repetitive slabs.
The structure was optimized with the bottom two layers of stoichiometric Fe 2 O 3 fixed in the bulk lattice sites, and the top layer of Fe(OH) x and loaded Pt were fully chirped in the structure optimization and transition state calculations.Definition of adsorption energies of surface species: E ads = E adsorbate/ catalyst -E catalyst -E adsordate , with E adsorbate/catalyst , E catalyst and E adsordate representing free gas molecules, adsorbed Pt/Fe(OH) x , substrate Pt/Fe(OH) x and the system energy, respectively.

CO catalytic oxidation activity performance
The CO oxidation performance of the Fe(OH) x , Pt 2 /Fe(OH) x and Pt 2 -Fe(OH) x catalysts is shown in Figure 1A.The Fe(OH) x catalyst without active component Pt loading showed a poor CO catalytic performance, CO could only be completely oxidized at 220 °C.As the Fe(OH) x catalyst were loaded with 2 wt% Pt by two methods (Pt 2 /Fe(OH) x by Wet-impregnation: and Pt 2 -Fe(OH) x by one-pot reduction), CO oxidation was improved significantly.As can be seen, in particular Pt 2 -Fe(OH) x , which was obtained via the one-pot reduction method, showed the best CO catalytic activity (the CO conversion could reach 70% only at 30 °C).Furthermore, the CO catalytic oxidation efficiency of Pt a -Fe(OH) x (a = 0.5, 1 and 2) catalyst with different Pt contents was further evaluated, the results was shown in Figure 1B, the CO catalytic performance of the catalysts decreases when the loading of Pt is decreased, and the complete CO oxidation temperatures of Pt 1 -Fe(OH) x and Pt 0.5 -Fe(OH) x are 80 °C and 130 °C, respectively.Thus, the Pt 2 -Fe(OH) x is the most potential CO catalytic oxidation catalyst.

CO catalytic oxidation stability evaluation
The H 2 O resistance and long-time stability of the catalysts are crucial, and the stability test of Pt 2 -Fe(OH) x catalyst is exhibited in Figure 2. As shown in Figure 2A, the Pt 2 -Fe(OH) x catalyst has a high initial CO catalytic oxidation activity at 50 °C (the CO conversion reaches about 90%), while the CO catalytic activity decreases significantly with the extension of the reaction time, and after 6 h, the CO conversion decreases to about 14%.Interestingly, when 3.0 vol% of H 2 O was introduced into the reaction gas, the CO catalytic activity was sharply enhanced, and the CO conversion increased from 14% to about 90% within a short time, and the CO catalytic activity remained unchanged within 10 h.When the H 2 O in the reaction gas was withdrawn, the CO conversion decreased steeply to about 20% after 6 h, and then the CO conversion rate decreased to about 20% after 3.0 vol% of water vapor was introduced into the reaction gas again.When 3.0 vol% of H 2 O was introduced to the reaction system, the CO catalytic activity of Pt 2 -Fe(OH) x catalyst was restored to the initial state, and the catalytic performance remained similar after five reaction cycles.The obtained results showed that water vapor significantly promoted the catalytic CO catalytic oxidation performance of (A) Fe(OH) x , Pt 2 /Fe(OH) x and Pt 2 -Fe(OH) x , (B) Pt a -Fe(OH) x (a = 0.5, 1, and 2) catalysts.

XRD, TEM and vacuum infrared spectroscopy analysis
The obtained XRD results of the catalysts are shown in Figure 3.It can be seen from Figure 3A that the Fe(OH) x , Pt a -Fe(OH) x and Pt 2 /Fe(OH) x catalysts after drying at 150 °C all showed the characteristic diffraction peaks of Fe(OH) 3 , which indicated that the catalyst carriers mainly existed in the form of Fe(OH) 3 at this temperature.Figure 3B shows the XRD spectrum of the catalyst after the reaction, the catalyst still exhibits the typical characteristic diffraction peaks of Fe(OH) 3 , which indicates that the composition of the catalyst does not change.In addition, the characteristic diffraction peaks for Pt were not detected for the fresh Pt-containing catalysts, which was mainly attributed to the low or highly dispersed Pt active component, and the characteristic diffraction peaks for the reacted catalysts were not detected, which indicated that the Pt species did not grow up agglomerated.Fe(OH) 3 has a large number of hydroxyl species, and the Pt species can be highly dispersed on Fe(OH) 3 without sintering and aggregation for a long time, which is an important factor for the excellent catalytic performance of the Pt 2 /Fe(OH) x catalysts prepared by the one-pot reduction method.
Transmission electron microscopy (TEM) characterization of the Pt 2 -Fe(OH) x catalyst is shown in Supplementary Figure S1.For the Pt 2 -Fe(OH) x catalyst prepared by the one-pot reduction method, a lattice fringe of 0.23 nm was detected, which corresponds to the (111) crystalline plane of the Pt metal; however, no lattice fringe was detected with respect to the Pt species for the Pt 2 -Fe(OH) x catalyst prepared by the impregnation method.This suggests that the preparation method influences the form of Pt species present in the catalyst, which in turn influences the catalytic activity of the catalyst.
The vacuum infrared spectroscopy of fresh and used Pt 2 -Fe(OH) x catalysts is shown in Supplementary Figure S2, in which the peak at 3,389 cm −1 is corresponding to characteristic peak of hydroxyl species on the catalysts surface.It can be seen that the intensity of the peak of the hydroxyl species of the catalyst decreased significantly after the reaction, which indicates that the hydroxyl species were gradually consumed with the reaction, which is the main factor for the decrease of the catalyst performance.When 3.0 vol% H 2 O was introduced into the reaction gas, it was beneficial to compensate for the depleted hydroxyl species on the catalyst surface, so the catalytic activity of the catalyst could be recovered rapidly.

XPS and H 2 -TPR results analysis
The CO catalytic oxidation reaction takes place on the catalyst surface, so it is crucial to explore the chemical properties and composition information of surface species.The obtained Pt 4f orbitals XPS spectra of the catalysts are shown in Figures 4A, B, and the presence of Pt 0 and Pt 2+ can be detected for the Pt 2 -Fe(OH) x (Pt 0 / Pt ratio is 0.69) prepared by the one-pot reduction method and for the Pt 2 /Fe(OH) x (Pt 0 /Pt ratio is 0.30) catalysts prepared by the Frontiers in Chemistry frontiersin.orgimpregnation method (Oi-Uchisawa et al., 2003;Rohrbach et al., 2004), and the quantitative results, as shown in Table 1, indicate that Pt 2 -Fe(OH) x has the most Pt 0 , i.e., the one-pot reduction method is favorable for the reduction of Pt species, which is consistent with the TEM results of the catalysts.Moreover, numerous studies have shown that Pt 0 is more favorable for the catalytic oxidation of CO compared with Pt 2+ .As shown in Figure 4C, the catalyst surface exhibited two kinds of oxygen species (the surface adsorption oxygen species (O ads ), which are mainly hydroxyl species, and the lattice oxygen (O lat ) species) (Si et al., 2020;Su et al., 2020;Xiong et al., 2020;Yang et al., 2020).Quantitative results show that Pt 2 -Fe(OH) x has the most surface oxygen species (the O ads /O lat ratio is 2.11), higher than that of Pt 2 /Fe(OH) x (the O ads / O lat ratio is 1.98), indicating that it has the most surface hydroxyl species, which is consistent with the best CO catalytic oxidation activity.The results of the H 2 -TPR curves of the catalysts are shown in Figure 4D, with the reduction peaks at 90-150 °C attributed to the reduction of Pt species, and the reduction peaks at 300-600 °C attributed to the reduction of Fe(OH) 3 species.Compared with  the carrier Fe(OH) x , the Fe(OH) 3 of Pt 2 -Fe(OH) x species prepared by the one-pot reduction method was shifted towards lower temperatures, suggesting that the introduction of Pt could improve the oxidation capacity.In addition, the reduction peak temperature of the Pt element in Pt-Fe(OH) x was shifted to the low temperature direction compared with that of the Pt 2 /Fe(OH) x catalyst prepared by impregnation method, which indicated the improvement of the oxidation capacity of the catalyst, and this was one of the reasons for the better CO catalytic oxidation performance of the Pt 2 -Fe(OH) x catalyst prepared by the one-pot method.
Fe 2 O 3 surface to be E ads = −117.7 KJ/mol, which is consistent with the DFT calculation results for the formation of H 2 .The theoretical calculations indicate that there is a water-gas reaction on the Pt-Fe(OH) x surface, which leads to the depletion of surface -OH.Chen et al. (2018) experimentally concluded that the Pt-FeO x surface undergoes water-gas reaction.

Conclusion
In this work, the prepared Pt a -Fe(OH) x catalysts (a represents the mass fraction of Pt loading (%), a = 0.5, 1 and 2) by the one-pot reduction method exhibited excellent CO catalytic activity, with the Pt 2 -Fe(OH) x catalyst, 70% and ~100% CO conversion was achieved at 30 °C and 60 °C, respectively.In addition, the Pt 2 -Fe(OH) x catalyst also showed excellent H 2 O resistance and hydrothermal stability in comparison to the Pt 2 / Fe(OH) x catalyst prepared by impregnation method.Characterization results showed that the excellent catalytic performance of the catalysts was mainly attributed to the abundant surface oxygen species and Pt 0 the presence of H 2 O, which promoted the catalytic reaction of CO.A combination of in situ DRIFTS and DFT calculations was used to establish the detailed catalytic CO removal pathway on Pt-Fe(OH) x surface, that is: ① 2 CO free →CO free + CO ads , ② CO free + CO ads + OH → COOH ads + CO free , ③ COOH ads + CO free → CO 2ads + H ads + CO free , ④ CO 2ads + H ads + CO free → CO 2ads + H ads + CO ads , ⑤ CO 2ads + H ads + CO ads + OH → CO 2ads + H ads + COOH ads , ⑥ CO 2ads + H ads + COOH ads → 2CO 2ads + 2H ads , ⑦ 2CO 2ads + 2H ads → 2CO 2free + 2H ads and ⑧ 2CO 2ads + 2H ads → 2CO 2free + H 2free .Two CO and -OH on the surface of Pt-Fe(OH) x form -COOH with reaction barriers of 0.83 and 0.65 eV, respectively, which indicates that the -OH on the surface of Pt-Fe(OH) x has a better activity and can easily oxidize CO to -COOH, which can be further detached to CO 2 and H atoms, with reaction barriers of 0.77 and 0.65 eV, respectively.The existence of H 2 O reaction on the surface of Pt-Fe (OH) x that H 2 O decomposes into H and -OH, and the combination of -OH and CO into -COOH leads to the depletion of the -OH on the surface of Pt-Fe(OH) x .If there is no H 2 O added continuously during the reaction, the -OH is not replenished, and there is no -OH to participate in the oxidation of CO, the catalyst activity will be decreased, which is consistent with the activity of our catalyst, i.e., the activity is decreased when we withdraw H 2 O, and the activity is recovered when -OH is replenished after H 2 O is added.This study holds significance for further understanding the application potential of nano-catalysts in important oxidation reactions and provides valuable insights for the development of efficient CO oxidation catalysts.

FIGURE 2 CO
FIGURE 2 CO catalytic oxidation performance with 3.0 vol% H 2 O of Pt 2 -Fe(OH) x catalyst (A) five reaction cycles, (B) stability of Pt 2 -Fe(OH) x catalyst at 50 °C.
FIGURE 4 (A-C) XPS spectra and (D) H 2 -TPR results of catalysts.

TABLE 1
Quantitative XPS peak results of the catalysts.